Two immune systems, innate and acquired, are known in a living body. In contrast to acquired immunity, which is found only in higher organisms such humans, the innate immune system is conserved from insects to humans in a wide range of organisms. When external pathogenic bacteria or such invade, cells responsible for the innate immunity (for example, macrophages and dendritic cells) function as the first-stage prevention of infections: quickly detect pathogenic bacteria or such, directly attack the pathogenic bacteria or such through phagocytosis or such, and release alarm signals such as cytokines to activate the acquired immune system. Toll-like receptors (TLRs) play a role in the first detection of bacteria in a series of immune responses. Ten types of TLRs have been identified in human so far, each being considered a receptor recognizing a different molecular structure (Non-Patent Document 1). Of these, TLR9, which was first reported by the research group of Akira et al. in 2000, was identified as a receptor that recognizes bacterial DNA, in particular the CpG motif (Non-Patent Document 2 and Patent Document 1).
Previous studies have reported the effect of lipopeptides on the expression activity of NF-κB in human TLR2-expressing CHO cells, as well as the effect of various CpG DNA motifs derived from pathogenic E. coli on cytokine (IL-8) yield in human TLR9-expressing HEK293 cells (Non-Patent Documents 3 to 5).
Information on prior art documents relevant to the invention of the present application is listed below.    [Patent Document 1] Japanese Patent Application Kokai Publication No. (JP-A) 2002-34565 (unexamined, published Japanese patent application)    [Non-Patent Document 1] O. Takeuchi, S. Akira, International Immunopharmacology 1 (2001) 625-635    [Non-Patent Document 2] H. Hemmi et al., Nature 408 (2000) 740-745    [Non-Patent Document 3] Yoshimura A., Takada H., Kaneko T., Kato I., Golenbock D., Hara Y., “Structural requirements of muramylpeptides for induction of Toll-like receptor 2-mediated NF-kappaB activation in CHO cells.” Journal of Endotoxin Research 6 (5): 407-20, 2000.    [Non-Patent Document 4] Klinman D M., Takeshita F., Gursel I., Leifer C., Ishii K J., Verthelyi D., Gursel M., “CpG DNA: recognition by and activation of monocytes.” Microbes & Infection 4 (9): 897-901, 2002 July.    [Non-Patent Document 5] Takeshita F., Leifer C A., Gursel I., Ishii K J., Takeshita S., Gursel M., Klinman D M., “Cutting edge: Role of Toll-like receptor 9 in CpG DNA-induced activation of human cells.” Journal of Immunology 167 (7): 3555-8, 2001 Oct. 1.